This invention relates to the field of digital logic circuits and sequential state machines.
Digital logic circuitry has proved generally useful for performing logical functions and transformations which may be required in the automated control of complex hardware. However, while a given logic circuit is adept at performing a particular specialized function, few logic circuits are capable of use in anything other than one specialized function. It is therefore generally required that a new logic circuit be designed and fabricated for each new function which is desired, leading to added cost of design and fabrication, and to reduced reliability as new logic circuits may fail due to "bugs" or errors.
In some applications, such as aerospace environments, the need for logic circuit control of complex hardware is acute, and the environment is generally intolerant of reliability errors. The cost of design and fabrication is exacerbated where there are many differing specialized functions to be performed, by the need to design each new logic circuit, by the "learning curve" experienced by process engineers in fabrication of each new logic circuit, and by reliability costs, e.g. testing and replacement, associated within each new logic circuit. Accordingly, there is a need for digital logic circuitry which is tailorable by software to perform any designated logic function.
A logic circuit which was capable of performing more than just one specialized function would obviate many of the problems noted above. Logic circuits which can perform any logic function, and which may be tailored by software to perform a specified function, are called "universal" logic circuits. Universal logic circuits are known by theory to fall into two classes: (1) "combinational" circuits, which may calculate any logic function of its inputs which does not require memory of earlier inputs, and (2) "sequential" circuits, which do have memory of earlier inputs. Construction of universal sequential logic circuits which are susceptible of fabrication with present technology has proved extremely difficult in the past. Accordingly, there is a need for a universal sequential logic circuit which is susceptible of fabrication with present technology.